1. Field of the Invention
The present invention relates to a semiconductor light-emitting diode (LED) with a short wavelength, which uses a compound semiconductor material having a wide band gap, and a method of manufacturing the same.
2. Description of the Related Art
As a consequence of the development of high-speed, high-density data processing systems, demand has arisen for the realization of an LED having a short wavelength; in particular, a blue high-luminance LED.
Group III-V compound semiconductor materials, from which a blue LED is expected, and having a wide band gap, include nitrides and phosphides of relatively light III group elements, such as BN (4 or 8 eV), AlN (6 eV), GaN (3.4 eV), InP (2.4 eV), AlP (2.5 eV), and GaP (2.3 and 2.8 eV). However, it is difficult to synthesize the high pressure phase (c-BN) having sp3 bonding. In addition, BN has three types of polymorphism and tends to form a mixture. Furthermore, it is difficult to perform impurity doping in the case of BN. For the above reasons, therefore, BN is not suitable. InN has an insufficiently wide band gap for the realization of a blue LED and poor thermal stability, and only a polycrystal can normally be obtained. AlP and GaP both have insufficiently wide band gaps. The remaining materials, AlN and GaN, both have wide band gaps and excellent stability, and are therefore are suitable for short-wavelength light emission. However, AlN and GaN have a Wurzite type crystal structure (to be referred to as a WZ type hereinafter). In addition, since they have a high ionizing property, lattice defects easily occur, and a p-type semiconductor with low resistivity cannot be obtained.
Accordingly, attempts have been made to mix B and N with a III-V group-based compound which has been developed as a material for a conventional semiconductor laser, to obtain a material having a wide band gap and which does not have the above drawbacks. However, the lattice constant of the conventional material differs from that of the material containing B and N by as much as 20 to 40%, and both materials also have different crystal structures. Consequently, a stable crystal cannot be obtained. For example, when N is mixed with GaP, the ratio of N to GaP was 1% or less. Thus, a sufficiently wide band gap material cannot be obtained.
According to studies carried out by the present inventors, a p-type crystal having a low resistance cannot be obtained using GaN or AlN, due to their high ionizing properties, defects can easily occur. However, the essential reason for their non-usability is that GaN and AlN do not have a zinc blend type crystal structure (to be referred to as a ZB type hereinafter), but rather a WZ structure.
As described above, no conventional semiconductor material can fully satisfy the conditions that the band gap required to realize a high-luminance blue LED is as large as, for example, 2.7 eV or more, and that p- or n-type conductivity can be controlled, and excellent crystal quality can be achieved. Although a nitride such as AlN or GaN is a suitable material for obtaining a large band gap, a p-type layer with low resistivity cannot be obtained using such a nitride.